This invention relates to the production of printing plates and, more particularly, to a method and apparatus for separating and releasing a curved flexible printing plate after its formation in a mold.
Recently, there have been developed flexible materials that offer great promise as a replacement for rigid metal printing plates. These flexible materials are plastic. Flexible plastic printing plates are lighter and cheaper than metal plates, have a longer press life, and provide a more accurate representation of the matter to be printed. Until now, flexible plastic printing plates have been produced in a flat mold, as illustrated in U.S. Pat. No. 3,743,463, which issued July 3, 1973 to Richard B. Patrick and John Sonia. The thought process of those working in the art seems to have been that flexible flat plates can be bent easily enough to conform to the curved surface of the cylinder. The flexible plates in the prior art are, therefore, flat in their unstressed condition and become stressed when bent to conform to the curvature of the plate cylinder on which they are mounted.
Flexible printing plates that are flat in their unstressed condition must be held under great longitudinal tension by the cylinder attaching members in order to make the plate fully conform to the cylinder over its entire surface. If insufficient longitudinal tension is applied to the plate, background areas on the plate are falsely printed. If too much longitudinal tension is applied to the plate, the matter to be printed becomes distorted and the plate may creep, i.e., become permanently deformed, and satisfactory color registration cannot be established.
Newspaper printing presents special problems that must be overcome before the use of flexible printing plates becomes practical. In the course of the daily operation of newspaper presses, many printing plates must be installed on and removed from the plate cylinders to print the pages of the various editions. For example, a major newspaper may require a half million or more printing plates a year. Accordingly, the cylinder attaching arrangement must permit fast, easy installation and removal of the printing plates. Further, it is typical for editions with large distribution that a number of presses simultaneously print the same pages, so multiple printing plates, as many as fifteen or more, are then required.
My application Ser. No. 478,826, filed June 12, 1974, now abandoned, teaches that a flexible printing plate should be molded with the same curvature as the cylinder on which the plate is mounted. Thus, the plate is unstressed when it conforms to the surface of the cylinder and no longitudinal bending strain results. To attach the printing plate to the cylinder, inwardly bent, laterally extending hooks at the ends of the plate engage projections on the cylinder.